Metal-tolerant Pantoea sp. WP-5 and organic manures enhanced root exudation and phytostabilization of cadmium in the rhizosphere of maize

This study investigated the phytoremediation potential of maize (Zea mays L.) in Cd-contaminated soil through co-inoculation of metal-tolerant plant beneficial rhizobacteria (MtPBR: Pantoea sp. strain WP-5) with organic manures (PM, poultry manure, and BGR, biogas residues). The objectives of this study were to (i) examine comparative efficiency of MtPBR, PM and BGR alone or in combined form to improve maize biomass and physiology and (ii) understand the role of organic acid production in root exudates of maize for Cd accumulation and translocation. Pantoea sp. WP-5 showed organic acid production and tolerance to high Cd concentration (1000 mg L^-1), thereby inoculated to maize seeds sown in soil spiked with 75 mg Cd kg^-1 soil and 500 g each of the organic manures per pot. The co-inoculation of MtPBR + BGR significantly (P<0.05) increased chlorophyll contents, root/shoot dry weight, photosynthetic rate, stomatal conductance, and relative water contents and decreased electrolyte leakage, malondialdehyde contents, ascorbate peroxidase, and catalase activity in maize over the control treatment. The co-inoculation of MtPBR + BGR produced significantly (P<0.05) higher concentrations of acetic and citric acid (52.7±0.5 and 22.8±0.08 μg g^-1 root fwt, respectively) in root exudates of maize, which immobilized Cd within plant roots inferred by the positive relation (root Cd vs. organic acids; R² = 0.80-0.92) and reduced Cd translocation to shoots inferred by the negative relation (shoot Cd vs. organic acids; R² = 0.81-0.90). It is concluded that the application of MtPBR + BGR enhanced organic acid induced phytostabilization and accumulation of Cd in roots and restricted its translocation to shoots.